Synthesis of Neem Bark Based Nanoadsorbent for Efficient Toluidine Dye Removal: A Comprehensive Kinetic, Isotherm, and Thermodynamic Investigation

In this study, the adsorption behavior of the cationic organic dye Toluidine Blue (TB) from aqueous solution was explored using a nanoadsorbent derived from the dead bark of Azadirachta indica (neem). The synthesized activated carbon was characterized using FTIR, zeta potential analysis, UV-Vis spectroscopy, FESEM, BET, and TEM techniques. The N₂ adsorption–desorption analysis revealed a type IV isotherm with an H3 hysteresis loop, indicating the formation of mesoporous structures with pore diameters ranging from 2–50 nm. Batch adsorption experiments were conducted to evaluate equilibrium isotherms, investigating the influence of key parameters such as contact time, pH, temperature, initial dye concentration, and adsorbent dosage. Under optimized conditions—20 mg/L initial dye concentration, pH 12, and 20 mg adsorbent dose—a maximum removal efficiency of 98.76% was achieved within 70 minutes. However, at higher initial dye concentrations (60 mg/L), the removal efficiency decreased to 82.28% under similar conditions. The adsorption data aligned well with the Langmuir isotherm model, as evidenced by a high correlation coefficient (R² = 0.999), and the maximum adsorption capacity was estimated to be 106.92 mg/g at 303 K. Kinetic analysis confirmed that the adsorption followed a pseudo-second-order model, and although intra-particle diffusion contributed to the process, it was not the sole rate-limiting step. Thermodynamic parameters indicated the spontaneous and endothermic nature of the adsorption, with negative ΔG°, positive ΔH°, and positive ΔS°, suggesting increased randomness at the solid–liquid interface during dye uptake.